Morphological and immunohistochemical studies of the lungs and bronchus-associated lymphoid tissue in a rat model of chronic pulmonary infection with Pseudomonas aeruginosa.

Pseudomonas aeruginosa is one of the most frequently encountered bacterial pathogens in patients with chronic pulmonary infections, including cystic fibrosis and diffuse panbronchiolitis. Bronchus-associated lymphoid tissue (BALT), noted frequently in patients with cystic fibrosis and diffuse panbronchiolitis, is considered to play an important role in the local immunologic defense mechanisms in the respiratory tract. To investigate the role of BALT in chronic pulmonary infections, we developed an animal model for chronic pulmonary infection and studied the morphological and immunohistochemical characteristics of BALT. Experimental pneumonia was produced in rats by intratracheal inoculation of P. aeruginosa enmeshed in agar beads. The histological changes corresponded to those occurring in chronic bronchiolitis. Immunohistochemically, surface immunoglobulin M-positive (sIgM+) cells and sIgA+ cells were recognized in the inflamed bronchial walls from day 4, and sIgG+ cells were recognized from day 14, W3/25+ cells exceeded OX8+ cells in number until day 14. In the BALT, there was a massive accumulation of lymphocytes in the lymphatics and high endothelial venules. The development of germinal centers was accompanied by increased numbers of sIgM+ and sIgA+ cells. W3/25+ cells exceeded OX8+ cells in number in the BALT until day 14. On the other hand, OX8+ cells were predominant in comparison with W3/25+ cells at day 21, and then both sIgM+ and sIgA+ cells and inflammatory changes in the lung decreased at day 28. These findings suggest that BALT regulates the local immune responses against chronic pulmonary infection due to P. aeruginosa.